Spring cores are widely used in seating or bedding products. Such spring cores commonly are made from a matrix of multiple springs joined together directly as by helical lacing wires, or indirectly as by fabric within which each individual spring is contained. Pocket spring cores in which springs are respectively contained in a pocket of fabric are popular, due to the comfort and luxury feel provided by pocket spring cores.
In order to provide firm support, it is desirable to use springs having a high firmness. This can be attained by preloading springs. U.S. Pat. No. 6,186,483 B1 and U.S. Pat. No. 5,924,681 B1 respectively describe springs having knotted end turns, in which the spring is preloaded using a loop of fabric.
U.S. Pat. No. 4,817,924 describes a spring core for a mattress in which springs have unknotted end turns. The end turns include portions which essentially extend perpendicular to a longitudinal axis of the spring. Other examples for coil springs having unknotted end turns are described in US 2010/0295223 A1 and U.S. Pat. No. 7,921,561 B1, for example. The flat surface defined by the end turns of the springs, even in the rest state of the springs in which the springs are unloaded, assists in providing a flat support surface, which is desirable in terms of comfort.
Springs for use in pocket spring cores have traditionally been designed so as to define an end surface oriented normal to the spring axis in the rest state of the spring. Frequently, the end turns are knotted. By using springs having end turns with ring-like portions oriented perpendicular to the longitudinal axis of the spring, flat surfaces may be defined at the upper and lower ends of the spring. Such ring-like support surfaces assist in providing the pocket spring core with comparatively flat upper and lower surfaces. Further, problems associated with wear of the pocket material may be mitigated.
While high comfort and luxury feel can be attained by using springs that have flat end turns oriented normal to the spring axis, the flat end turns do not contribute to the firmness of the spring. Thus, such spring configurations may require a greater amount of wire. To provide greater firmness while reducing the overall wire length, a more aggressive pitch could be used on the central portion of the spring. However, in order for the spring to retain its shape memory, there are bounds for the pitch which can be used. The greater amount of wire required for producing the springs used in conventional pocket spring cores increases the costs of such spring cores.